netdev - Re: bond: take rcu lock in bond_poll

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Message-ID: <20180928182105.ocenua7bftli7q7x@codemonkey.org.uk>
Date:   Fri, 28 Sep 2018 14:21:05 -0400
From:   Dave Jones <davej@...emonkey.org.uk>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: bond: take rcu lock in bond_poll_controller

On Fri, Sep 28, 2018 at 10:31:39AM -0700, Cong Wang wrote:
 > On Fri, Sep 28, 2018 at 10:25 AM Dave Jones <davej@...emonkey.org.uk> wrote:
 > >
 > > On Fri, Sep 28, 2018 at 09:55:52AM -0700, Cong Wang wrote:
 > >  > On Fri, Sep 28, 2018 at 9:18 AM Dave Jones <davej@...emonkey.org.uk> wrote:
 > >  > >
 > >  > > Callers of bond_for_each_slave_rcu are expected to hold the rcu lock,
 > >  > > otherwise a trace like below is shown
 > >  >
 > >  > So why not take rcu read lock in netpoll_send_skb_on_dev() where
 > >  > RCU is also assumed?
 > >
 > > that does seem to solve the backtrace spew I saw too, so if that's
 > > preferable I can respin the patch.
 > 
 > 
 > >From my observations, netpoll_send_skb_on_dev() does not take
 > RCU read lock _and_ it relies on rcu read lock because it calls
 > rcu_dereference_bh().
 > 
 > If my observation is correct, you should catch a RCU warning like
 > this but within netpoll_send_skb_on_dev().
 >
 > >  > As I said, I can't explain why you didn't trigger the RCU warning in
 > >  > netpoll_send_skb_on_dev()...
 > >
 > > netpoll_send_skb_on_dev takes the rcu lock itself.
 > 
 > Could you please point me where exactly is the rcu lock here?
 > 
 > I am too stupid to see it. :)

No, I'm the stupid one. I looked at the tree I had just edited to try your
proposed change. 

Now that I've untangled myself, I'll repost with your suggested change.

	Dave